The present invention relates to the field of endodontic instruments adapted for use in performing root canal therapy on teeth, and which are characterized by high flexibility and high resistance to torsional breakage.
Root canal therapy is a well-known procedure wherein the crown of a diseased tooth is opened so as to permit the canal to be cleaned and then filled. More particularly, after opening the crown, a series of very delicate, flexible, finger-held instruments or files are used to clean out and shape the root canal, and each file is manually rotated and reciprocated in the canal by the clinician. Files of increasingly larger diameter are used in sequence, to achieve the desired cleaning and shaping. When the canal is thus prepared, it is solidly filled with a filling material, which typically comprises a waxy, rubbery compound known as gutta percha. In one procedure, the gutta percha is positioned on an instrument called a compactor, and the coated compactor is inserted into the prepared canal and rotated and reciprocated to compact the gutta percha therein. The clinician then fills the tooth above the gutta percha with a protective cement, and lastly, fits a crown to the tooth.
Endodontic files of the described type are commonly supplied to the clinician in kits which comprise several files of increasing diameter. In particular, and in accordance with ANSI/ADA Specification No. 28-1988, files are provided in diameters which range from 0.08 mm at the tip (size 08) to 1.40 mm at the tip (size 140), and the files are provided in kits which contain a number of files of increasing diameter so that the files from a particular kit may be used in sequence by the clinician in accordance with the requirements of the particular canal being cleaned.
As is well-known by clinicians, procedural errors occasionally occur during root canal therapy. These errors result in changes to the root canal morphology such as the perforation of the canal, and the formation of a ledge in the wall of the canal. Also, the instrument may fracture. These errors usually occur because the endodontic instruments, which are typically formed of stainless steel, lack the requisite flexibility particularly in the larger diameter instruments. Thus, upon being inserted into the root canal of a tooth, the stainless steel instrument is often unable to conform to the sometimes compound curvature or outline of the canal. In particular, the instrument tends to cut into the inside edges of the curved canal (referred to as transportation), which can lead to the perforation of the wall of the canal, and ledges may be formed at the pilot end of the instrument by reason of a lateral movement of the pilot end.
Depending upon the severity of these errors, repair measures might be needed. For instance, if a perforation of the wall results, the perforation must be repaired to prevent bacteria from collecting and the possibility of saliva leaking into the cavity.
Recently, endodontic files composed of a nickel-titanium alloy have been introduced, which provide a high degree of flexibility in both bending and torsion, and superior resistance to fracture, as compared to stainless steel instruments. In this regard, reference is made to the article entitled "An Initial Investigation of the Bending and the Torsional Properties of Nitinol Root Canal Files", Journal of Endodontics, Volume 14, No. 7, July 1988, at pages 346-351.
While the new nickel-titanium instruments have been found to reduce transportation and the formation of ledges in the canal, in the smaller sizes, the highly flexible nature of the instruments reduces their filing or cleaning efficiency. Also, the tips of the instruments tend to deflect back upon themselves in severely curved canals.
It is accordingly an object of the present invention to provide an endodontic instrument which has a predetermined desired degree of flexibility for a given size or diameter of the instrument.
It is a further object of the present invention to provide a kit comprising a plurality of endodontic instruments wherein each instrument in the kit exhibits a predetermined desired degree of flexibility, and with the flexibilities of the instruments being chosen so as to provide an effective and predictable operational result for each of the instruments in the kit.
It is a more particular object of the present invention to provide a kit of a plurality of endodontic instruments of progressively increasing diameter, and wherein the instruments of smaller diameter have sufficient stiffness for effective filing and cleaning of the canal, and the instruments of larger diameter have sufficient flexibility to minimize transportation and ledge formations.